I was at school in Australia when Kevin was in his heyday (along with Rodney Rude). We knew all the words to all the songs, despite not being able to listen to them at home in case our parents heard what we were listening to.

Even some of our floors that were screwed down squeak (although far less than the ones that didn't get screws).
The glue is the key, imo. I believe Egger board suggest only fixing one end of their boards with screws and using adhesive-only everywhere else.

One thing that jumps out at me is the distance between the walk-in wardrobe and the ensuite in the master bedroom. Get out of bed, walk to the wardrobe to get out some clothes, walk all the way back around the bed to the ensuite. In an ordinary bedroom this would be bad enough, but at this scale, I know it would drive me mad in short order. We have a walk-through wardrobe, which we find very effective.
Along the lines of what @Ferdinand said, do some thought experiments about living in the house. Think about an entire week's worth of activities - what you do and where you go when you get up, head to breakfast, get the kids ready, have friends around and need somewhere to put the kids while the adults talk, etc, etc, etc. And then take into account whether your current layout makes all of these activities efficient.
As an example, we wanted a mudroom/laundry at a side entrance where we could come in with wet dog and muddy boots. We got one, but we never really thought specifically about how we'd use the space. The result is that there's no proper space for shoe/boot storage, and it's really narrower than it needs to be to properly perform its function.

Agreed. We never boost due to showers and have the system set well below BR rates, but the steam and condensation clear very quickly.
I might boost for an hour if I'm cooking something smelly, but that's about it.

I think the person-in-the-street increasingly expects to see UFH in a new build. Everyone I know who's done a significant extension, refurb, or new build has included UFH.
Re: costings, I'll be interested to see how you get on. I'd happily pay more (a lot more, actually) for UFH. The heat it gives is far more even and pleasant than any radiator-heated house I've been in. We did actually heat our house for a period last year with a single 2kW electric column heater in the kitchen. Amazingly, it was fine even when it was very cold, but the temperature wasn't evenly spread through the house by any means. With a low energy house, how do you distribute heat if your heating requirements mean you can only have a very small number of radiators?
If you have a lot of solar gain in any particular area, don't underestimate the advantage of UFH pipework moving heat around the slab. We have a huge south-facing slider that gets quite a bit of solar gain in the shoulder months. Without running the UFH pump, I suspect that the floor beside the slider - several square metres of our dining area - would become uncomfortably hot.
There's nothing stopping you from getting someone else in to do the UFH heating pipework, or doing it yourself.

Open cell insulation won't be ideal as water vapour can get through it and condense on the ducts.
I have some 180 or 160 closed cell ducting left over from my own MVHR installation if you want it. Let me know and I'll go measure it.

Bear in mind that if MBC does it and something goes wrong, they're responsible for putting it right. If someone else does the UFH in MBC's slab and something goes wrong, you potentially have a fight over who caused the problem. I'd personally consider that worth at least some sort of a premium.